1. Field of the Invention
The present invention relates to a process for the treatment of substrates using ions from a low voltage arc discharge. The process is used to prepare objects for a coating process, and to influence the layer growth during the application of the layer. The objects are referred to below as substrates. The process according to the invention is suitable in particular for the treatment of three-dimensionally structured substrates, for example for the treatment of tools, machine components or items of practical use before and during coating with an anti-wear layer.
2. Discussion of Background
Anti-wear layers usually consist of nitrides, carbides or carbonitrides of the transition elements of the 4th, 5th and 6th groups of the Periodic Table. The term transition element is used to denote the elements of the subgroups in the 3rd, 4th and 5th periods. It is also customary to use dry lubrication layers, for example layers of molybdenum sulphide, as a cover layer over a hard-material layer. Hard carbon layers, preferably containing metal and hydrogen, are applied to machine components.
Vacuum coating processes, in which at least one layer-forming species of atom is brought into the treatment space by atomization from the condensed phase, are referred to as physical vapor deposition (PVD) processes. The most important PVD processes are vacuum evaporation and cathodic sputtering, hereafter abbreviated to sputtering. The reactive evaporation and sputtering processes defined in the next section are also counted as PVD processes, in spite of the chemical reactions which they involve.
It is customary to heat the substrates before coating, in order to remove adsorbed gases and vapors, but heating may lead to the production of oxides. Oxide skins with a thickness of the order of a quarter of the wavelength of light (about 0.1 mm) can be seen with the naked eye as so-called tarnishing colors.
In order to remove oxide skins and other invisible surface coats from the surfaces of the substrates and their holders, the substrates are bombarded with positively charged ions. The material at the surfaces is in this case removed by sputtering and distributed on the walls and the supports, as well as on the substrates. The balance for the substrates is negative, that is to say a reduction in weight can be measured after the ion bombardment. It is therefore legitimate to refer to this treatment as "etching". The expression "cleaning" would only be justified with the reservation that a thin mixed layer is produced by sputtering around the surface layers, the substrate material, and the layer material existing on the holders as a result of the previous batch. This layer seems to improve the adhesion of the subsequently vacuum evaporated layer. This etching is a treatment in the sense of the present invention.
In particular in mechanical applications, very great requirements are made not only of the adhesion, but also of the elasticity of the layer. These requirements can be satisfied according to the invention by ion plating (IP). We use this expression for treatment by bombardment of the growing layer during the coating with ions. It condenses the layer and leads to a quite desirable internal compressive stress in the layer. Ion plating is also a treatment in the sense of the present invention.
A low-voltage arc is a gas discharge between a hot cathode, arranged in a chamber which is separated from the anode space but connected to it through an opening, and an anode. During operation, a noble gas is continuously introduced into the hot cathode chamber, and the anode space is kept under a sufficient vacuum for the intended process by pumping.
A known process for the treatment of substrates by means of ions from a low-voltage arc discharge is described in German patent 2823876. It is an evaporation method, in which the material to be evaporated is bombarded with electrons from the low voltage arc discharge.
In this process, the material to be evaporated is connected as the anode of the low-voltage arc discharge. This makes the anode space the evaporation space. In the evaporation space, the discharge pinched through the opening is additionally guided onto the anode by a magnetic field, which also focuses it (in order to achieve a sufficient power density on the surface of the anode for the evaporation). The material which evaporates is contained as a liquid melt or as a subliming granulate in an upwardly open cooled crucible. The guiding and focusing is brought about by a magnetic field whose field lines run parallel to the discharge and guide the discharge onto the crucible from above. This magnetic field is usually produced by a magnetic coil or pair of magnetic coils. The axes of these coils are in this case usually directed perpendicularly onto the crucible.
The substrate support customarily consists of rotatable substrate holders, or "trees". They are arranged around a circle in such a way that they form an axisymmetric cavity. The path of the magnetically focused low-voltage arc discharge runs through this cavity, predominantly along the axis. With an axisymmetric arrangement of this type, it is enough to rotate the trees about their own axes in order to achieve the uniform treatment. It is not necessary to rotate the entire substrate support about the axis of the cavity in carousel-fashion. However, if the intention is for it to be possible to use sources whose action is not axisymmetric, for example arc sources or cathodic sputtering sources located outside the cavity, then it is known to have the trees rotate both about the aforementioned carousel axis and about the tree axis (see, for example, EP 306612).
The process according to DE 2823876 is suitable for the production of nitrides, carbides or an carbonitrides by activated reactive evaporation (ARE) from metal vapor. The latter is activated by the electrons of low-voltage arc discharge to such an extent that, from the outset, the pure metal is evaporated and all nitrogen and/or carbon atoms are drawn from a gas. If ARE is combined with IP, then this results in activated reactive ion plating (ARIP). This involves more than just extra advantages, since not only noble gas ions, but primarily ions of the coating material are available for the ion plating.
The advantage of the known process is, last but not least, that it allows ion plating with metal ions and ion etching with noble gas ions. During etching, the substrates are preferably bombarded with argon ions, which are produced in the low-voltage arc discharge. By applying a negative bias voltage, ions diffusing out of the plasma of the low-voltage discharge are accelerated in an electric field in direction of the substrates.
The disadvantage of the known process is the fact that it is limited to metals which evaporate or sublime at temperatures below about 1700.degree. C. Titanium and chromium are the most important metals which can be evaporated from a crucible using a low-voltage arc. A further disadvantage of this process is that it is restricted to pure metals. The stoichiometric evaporation of alloys, for example titanium/aluminum for the production of TiAlN layers, leads to difficulties.
In terms of their advantages and disadvantages, arc sources are comparable with low voltage arc sources. Although they can evaporate alloys and make it possible to ion-plate with metal ions, they nevertheless only make it possible to etch with metal ions. Etching with noble gas ions has, however, proved unequivocally superior to etching with metal ions. The successful introduction of ternary and anti-wear layers made of alloys such as TiAlN brought with it the desire to equip arc source systems with a low-voltage arc for etching, in order to combine the advantages of these methods. A combination of this type, and its advantages over conventional metal ion etching, are described in the utility model DE 29615190.
According to DE 29615190, the low-voltage arc discharge does not penetrate the central cavity, formed by the substrates, but burns in a special discharge chamber on the outer wall of the treatment chamber. An arrangement of this type is intended to avoid limiting the economic viability of the above-described process according to DE 2823876. Quote: "This restriction is due to the fact that the low-voltage arc discharge, which penetrates the treatment chamber centrally, takes up a certain amount of space for itself, and in order to keep high-quality reproducible results, the workpieces need to have a corresponding separation from the discharge, with the result that a fairly large part of the central space of the treatment chamber cannot be used." A separation of from 15 to 25 cm is recommended for the novel etching device.
The etching device according to DE 29615190 has a great disadvantage in that the improvement in economic viability which is aimed for in the utility model is not achieved in practice. The reason is the risk of sparks being produced and leaving behind trails on the substrates. If the intention is to avoid defects of this type, only an ion current of limited strength should be applied to the substrates.